Researchers led by Katherine Flegal, an epidemiologist at the US Centers for Disease Control and Prevention, examined data from 97 studies with a total of 2.88m participants. More than 270,000 people died during the studies.
They found that those who were significantly obese, with a body mass index (BMI) of 35 or more, had significantly shorter lives than those who were normal weight, defined as having a BMI of 18.5 to 24.9.
However, those who were merely overweight, with a BMI of 25 to 29.9, died at a lower rate than those considered to be normal weight.
The overweight individuals were 6% less likely to die during the studies than those of normal weight. In addition, mildly obese people, defined as having a BMI of 30 to 34.9, were no more likely to die than those of normal weight.
“Our findings are consistent with observations of lower mortality among overweight and moderately obese patients,” the authors wrote.
They suggested that possible explanations for the findings could include heavier individuals receiving more thorough or earlier treatment for weight-related issues like high blood pressure or cholesterol; the possibility that increased body fat may protect the heart; and fat reserves helping to provide extra energy during severe illness.
However, some experts have criticised the study, suggesting that mortality is not a good measure of health and the statistics fail to take into account those who may be living longer in ill health.
Tam Fry of the UK’s National Obesity Forum said that the timing of the study’s publication was unfortunate, coinciding with a time of year when many individuals are resolving to improve their health by losing weight.
“Flegal has produced a paper which is being read by everyone as a one-size-fits-all, which is not the case,” Fry said.
He said that the statistics do not reflect people’s ability to carry a little extra weight as they get older, although this does not negate the idea that young people should try to be as lean as possible in order to prevent weight-related illness as they age.
“It is a good piece of work in my view. It has a good basis to it but the way it is being presented is confusing to many people,” Fry said.
The full study is available online here.